Masks

Learning goal

You will be able to apply an edit not to the whole photo, but only to an area you choose. You will explain that a mask is a map that decides where and how much an adjustment will be applied, that in white areas the adjustment is applied fully, in black areas not at all, and in gray areas partly. You will paint a face or an object by hand with a brush mask, separate the sky or the foreground with a soft transition with a gradient mask, use the AI-assisted masks where the program finds the sky and the subject itself, and with a range mask that selects by color or brightness, separate the areas HSL couldn't catch in the previous lesson. You will narrow masks by subtracting them from each other and intersecting them, check whether there are halos and spill at the edges, and check on different skin tones the mistakes AI-assisted masks make at hair and skin edges, correcting them by hand.

The question

The drummer shows a family portrait they shot in the living room on a holiday morning. Their mother is sitting right in front of the window, her face soft and bright. Their father, on the other hand, is beside her, a little behind, left in her shadow; his face is dark, his eyes hard to make out. When they raise the exposure, their father's face opens up, but their mother's face, the window and the white curtain blow out. The slider that opens the shadows also opens all the shadows in the frame together; the depth of the sofa and the rug is lost.

The same evening they also want to save their little sister's blue T-shirt in the balcony landscape from darkening along with the sky. In both photos what they want is the same: to make an adjustment in only one place. How do I brighten only a face? What is the way to tell the program "here" and "not there"?

Context

All the editing tools you have learned so far, the sliders in the basic panel, the tone curve and HSL, were applied to the whole photo. The tone curve let you choose which tones to change, HSL which colors, but neither knew position: they couldn't tell the sky's blue from the T-shirt's blue, or a face's tone from a wall of the same tone. In the darkroom, photographers did this by blocking the light with a piece of card or holding back particular areas with their hands.

Masks bring this choice of position to digital editing. Almost all of this module's following lessons, layers, cleanup, dodging and burning, skin retouching and color, rest on masks to some degree. This lesson tells what a mask is, the four basic kinds of masks, and the strengths and weaknesses of each.

Simple explanation

What is a mask?

A mask is a map that decides to which area of the photo an editing adjustment will be applied, and how much. You can think of a mask as a painting done between black and white: in places painted white the adjustment is applied fully, in places left black not at all, and in gray places partly, in proportion to the gray's lightness. Most programs show the mask as a semi-transparent colored overlay on the photo; where the overlay is, the adjustment is applied.

A mask's most important property is that its transitions can be soft. If the mask's edge is sharp while brightening a face, a line appears between the brightened area and its surroundings, and the edit is noticed at once. A mask whose edges soften slowly toward gray, on the other hand, blends the adjustment into its surroundings without giving it away. After you build a mask, you can add as many adjustments to it as you like: with the same mask you can raise the exposure, change the temperature and reduce the clarity all at once.

What are the kinds of masks?

A brush mask is the kind where the mask is created by painting it by hand with a brush. The brush's size, edge softness and flow are adjusted; a soft-edged brush keeps transitions natural, while low flow lets you build the mask up slowly over a few passes. The brush mask is the most flexible kind, but the one that takes the most effort; it is ideal for particular areas like a face, a hand or a small object. In most programs the same brush also has an erase mode; you can erase back the places that spilled.

A gradient mask is a mask that passes softly along a line from full effect to zero effect. A linear gradient is used to darken the sky or brighten the foreground; it is the editing counterpart of the graduated ND filter you met in the filters lesson. A radial gradient, on the other hand, settles around a face or a subject like a soft patch and directs the gaze there by slightly darkening the surroundings or brightening the center. The gradient mask's difficulty is that a straight line doesn't fit nature's non-straight boundaries: a gradient darkening the sky also darkens a tree or a tower rising above the horizon.

An AI-assisted mask is a mask where the program finds and selects the sky, the subject, a person, or parts like the face, hair or lips itself. These masks select, in seconds, complex edges that could take hours by hand. But they can be wrong: the edge of strands of hair, a translucent curtain, thin branches blending into the sky, or boundaries where subject and background are the same tone are the places where automatic selection goes wrong most often. You always need to enlarge an AI-assisted mask, check its edges and correct it with a brush if needed.

A range mask is a mask that selects an area not by position but by color or brightness. A color range mask selects, for example, only a particular blue; a luminance range mask only tones of a particular lightness. On its own this kind behaves like HSL; its real strength appears when it is combined with a mask that selects by position.

How are masks combined?

Masks can be added to each other, subtracted from each other and intersected. You can erase the top of a tree from a sky mask with a brush, or protect the person in front while darkening the sky by subtracting a subject mask from a gradient mask. When you intersect a person mask with a color range mask, on the other hand, you have selected only a particular color on that person. The drummer's little sister's blue T-shirt is saved this way: the adjustment darkening the sky is moved to a sky mask; because the T-shirt stays outside the sky, it isn't affected again.

The most common mistake at mask edges is a halo. When an area is brightened or darkened very differently from its surroundings, a thin, bright or dark line appears where the mask's edge doesn't sit exactly; it is seen most around a head or a building standing against the sky. Softening the adjustment a little, increasing the mask's edge softness, or correcting the edge finely with a brush removes this line.

AI-assisted masks can make different mistakes on different skin tones. When a dark-skinned person with black hair leans against a charcoal sofa or curtain, the subject mask can confuse the edge of the face and hair with the background; when the face is brightened, the ear, the jawline or the hair's edge stays left out, or part of the background is brightened too. When a light-skinned person stands in front of a bright window, on the other hand, the mask can confuse the face's edge with the window. The edges of curly, voluminous hair are also harder to select than straight hair. You need to enlarge the AI-assisted mask on every skin, check the edges and complete what is missing with a brush. While brightening a face, staying consistent with the photo's light direction, that is, not erasing the shadow side completely, keeps the face looking natural.

Visual evidence

In the drummer's family portrait you open a person mask; the program finds their mother and father separately. You select their father's mask and raise the exposure by one stop. On their father's dark skin his face opens up, and his eyes and the hairs of his mustache can be made out; their mother, the window and the curtain don't change at all. But when you enlarge the mask, you see that the edge of their father's ear and short curly hair has mixed with the dark sofa, and half of the ear has stayed in the dark. With a small soft-edged brush you add the ear and the hair's edge to the mask. Finally you pull the exposure increase back a little and leave a slight shadow on the side of his face away from the window; their father is now bright, but it is still clear that he is sitting in her shadow.

In the balcony landscape you undo the blue luminance adjustment that was darkening the sky, and apply the same adjustment to a sky mask. The sky deepens; the little sister's blue T-shirt returns to its old color. The top of a pine tree rising above the horizon has also got into the mask; you take it out with the brush's erase mode. A thin, bright halo has remained at the tree's edge; when you increase the edge softness, it disappears too.

Common mistakes

Summary

A mask is the map that decides to which area of the photo an adjustment will be applied and how much: fully in white, not at all in black, partly in the grays between, and soft edges blend the edit in without giving it away. A brush mask is painted by hand and most flexibly, a gradient mask builds a soft transition along a line, an AI-assisted mask finds the sky and the subject itself, and a range mask selects by color or brightness; by adding, subtracting and intersecting masks, the areas HSL couldn't catch are separated. Halos and spill at the edges are checked, the mistakes AI-assisted masks make at edges with dark skin and a dark background, light skin and a bright window, and curly hair are corrected with a brush, and the scene's light direction is kept on a brightened face.

Mini assignment

In this assignment you will try all four kinds of masks on the same photo; choose a photo with a person and a sky together.

1. Select a person's face first with a brush mask, then with an automatic person mask, and raise the exposure by half a stop; compare the edges of the two masks by enlarging them.
2. Darken the sky first with a linear gradient mask, then with an automatic sky mask; look at how objects rising above the horizon are affected.
3. Protect the person while darkening the sky by subtracting the person from the gradient mask.
4. By intersecting a color range mask with a person mask, change only the color of that person's clothing; at every step check whether there is a halo at the mask edges.

Check yourself

• Can you explain what a mask's white, black and gray areas mean?
• Can you decide in which situation to choose a brush, gradient, automatic or range mask?
• Did you enlarge and check the hair, ear and face edges of the AI-assisted mask on different skin tones?

Glossary

Mini quiz

1. How is an adjustment applied in a mask's gray area?

• A) Fully
• B) Partly, in proportion to the gray's lightness
• C) Not at all
• D) In the opposite direction

2. If a tower rising above the horizon also darkens while you darken the sky with a linear gradient mask, what is the most suitable correction?

• A) Remove the gradient mask
• B) Raise the exposure
• C) Crop out the tower
• D) Subtract the tower from the gradient mask with the brush's erase mode or a subject mask

3. Where do AI-assisted masks go wrong most often?

• A) On flat, single-color surfaces
• B) In the middle of the photo
• C) At the edges of strands of hair, in thin branches and at boundaries where subject and background are the same tone
• D) They never go wrong

4. On the face of a dark-skinned, dark-haired person brightened with an automatic person mask, the ear and the hair's edge stayed in the dark. Which is the most fitting correction?

• A) Enlarge the mask and add the missing edges to the mask with a soft-edged brush
• B) Raise the exposure even more
• C) Open the shadows of the whole photo
• D) Delete the mask and give up

Answers

1. B — Gray behaves like a proportion between white and black; the adjustment is applied to that degree.
2. D — Subtracting a mask corrects the gradient's straight line not fitting natural boundaries.
3. C — Thin, tangled edges and places where subject and background look alike make selection hard.
4. A — The edges the automatic selection left out are completed by hand; the rest stays in place.

Summary card

• A mask is a map: fully in white, not at all in black, partly in the grays between.
• Brush is flexible, gradient a soft transition, automatic fast, range by color.
• Add, subtract, intersect masks; separate what HSL couldn't catch.
• Check halos and spill at the edges, adjust the softness.
• Enlarge the automatic mask on every skin; keep the light direction while brightening.

Bridge to the next lesson

The mask with which you brightened their father's face was stored in the editing program as part of the photo's settings. When the drummer wanted to move the same portrait to another program to clean up a face and add some text, they noticed that some of their changes had been burned into the photo itself, with no way back. When they made a mistake, they had to start over. How do you edit without damaging the original? The next lesson tells about non-destructive editing, which keeps every change on a separate sheet with layers and adjustment layers.